The present invention relates to a d.c. voltage converter including a pulse width controlled semiconductor switch in the main primary circuit and a power transformer having at least three windings, with the primary winding of the power transformer being connected in series with the semiconductor switch and the secondary winding being connected via at least one rectifier element with a smoothing capacitor in the main secondary circuit. Such a d.c. converter is disclosed, for example, in Joachim Wustehube, Schaltnetzteile [Switching Network Components], published by Expert Verlag, 1979, page 410.
Switching transistors of clock pulse controlled d.c. voltage converters are operated by the load resistance with short or long turn-on pulses, depending on the load. It is customary to do this by comparing in a regulating circuit the integrated output voltage of the d.c. voltage converter with a reference voltage and deriving therefrom a regulating signal for controlling a pulse width modulator. Due to the integration of the output voltage required for trouble-free operation of the d.c. voltage converter, the d.c. voltage converter is able to react to changes in the load current only with a delay. This means that if the changes in the load current are rapid, the output voltage swings over or under to a greater or lesser degree, which is a particularly grave drawback in connection with d.c. voltage converters operating with small smoothing capacitances.
In order to save space and weight, d.c. voltage converters are now being developed which operate with higher switching frequencies permitting the use of small smoothing capacitances, e.g. foil capacitors, which have the advantage over aluminum electrolyte capacitors that they have a longer service life. Conventional d.c. voltage convertors are able to operate only marginally with small smoothing capacitances and only if no changes or only small changes can occur in the load current.
If, however, the jump in the load current is large, e.g. from maximum load to the complete absence of a load (idling), the stored energy of an inductance, for example the secondary inductance of the power transformer or the inductance of a smoothing choke, cannot be prevented from flowing into the smoothing capacitor, even if the switching transistors were switched off immediately. In this case the smoothing capacitor may take on a voltage which is a multiple of the normal output voltage.